1. Field of the Invention
This invention relates to means of stimulating electrically excitable tissue, and more particularly relates to means for adjusting the locus at which action potentials are induced in such tissue.
2. Description of the Related Art
Two major practical problems reduce the efficacy of epidural spinal cord stimulation (SCS) for pain control. One is the difficulty of directing the stimulation-induced paresthesia to the desired body part and the other is the problem of disagreeable sensations or motor responses to the stimulation, which reduce the comfortable amplitude range of the stimulation. It is generally agreed that in SCS, for chronic pain, paresthesia should cover the whole pain region. With present stimulation methods and equipment, only highly skilled and experienced practitioners are able to position a stimulation lead in such a way that the desired overlap is reached and desired results are obtained over time with minimal side effects. It requires much time and effort to focus the stimulation on the desired body region during surgery and, with single channel approaches, it is difficult to redirect it afterwards, even though some readjustments can be made by selecting a different contact combination, pulse rate, pulse width or voltage.
Redirecting paresthesia after surgery is highly desirable. Even if paresthesia covers the pain area perfectly during surgery, the required paresthesia pattern often changes later due to lead migration, or histological changes (such as the growth of connective tissue around the stimulation electrode) or disease progression. The problem of lead placement has been addressed by U.S. Pat. No. 5,121,754 by the use of a lead with a deformable distal shape. These problems are not only found with SCS, but also with peripheral nerve stimulation (PNS), depth brain stimulation (DBS), cortical stimulation and also muscle or cardiac stimulation.
A system capable of some adjustment of spinal cord excitation is described in PCT International Publication No. WO 95/19804 (counterpart to Holsheimer et al., U.S. Pat. No. 5,501,703). However, that system requires three electrodes, optimally spaced, which is a serious handicap during the surgical procedure required in order to place these electrodes in the body. That system steers the locus of excitation by varying the potentials between the electrodes that are optimally spaced in a line.
In fact, the electrodes in that prior art system are referred to as "in-line" electrodes that are disposed "symmetrically" along a line. The electrical field pattern, across that line is adjusted by varying the electrical field generated between those electrodes along that line. The locus of excitation is correspondingly varied with that variation in the electrical field pattern.
Thus, because U.S. Pat. No. 5,501,703 to Holsheimer et al. requires multiple electrodes optimally spaced symmetrically along a line, a lead such as a paddle is used for mounting the multiple electrodes in the optimally spaced positions. This lead is then inserted within a patient near the tissue to be excited with the electrical excitation applied to the lead. Unfortunately, placement of a lead such as the paddle within a patient, can be difficult since the paddle can be surgically difficult to manipulate adjacent the spinal cord.
Thus, it would be desirable to be able to adjust the locus of excitation in electrically excitable tissue without the use of optimally spaced electrodes.